Keston E01 Fault Code: Ignition Failure
The E01 code appears on Keston C36 boilers (note: the equivalent fault on C40 and C55 models is E02) when the boiler has made five consecutive attempts to light the burner without success. Rather than continuing to try indefinitely, the boiler enters a hard lockout to protect itself and your home. You must press the reset button on the control panel to clear the lockout — simply switching the boiler off and on at the mains will not work.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Keston E01 fault code mean?
The E01 code appears on Keston C36 boilers (note: the equivalent fault on C40 and C55 models is E02) when the boiler has made five consecutive attempts to light the burner without success. Rather than continuing to try indefinitely, the boiler enters a hard lockout to protect itself and your home. You must press the reset button on the control panel to clear the lockout — simply switching the boiler off and on at the mains will not work.
Common causes
- Frozen condensate pipe Common
During cold snaps, the external section of the condensate pipe — which carries waste water away from the boiler — can freeze solid. This blocks the condensate outlet, causing the boiler to shut down and triggering an ignition lockout. This is by far the most frequent cause of E01 in winter, particularly in flats where the external pipe run tends to be long.
- Gas supply interruption or low gas pressure Common
If there is insufficient gas reaching the boiler, it simply cannot ignite. On cold mornings, regulators on external gas meters can ice up and restrict flow. Check whether other gas appliances in the property (hob, gas fire) are working normally. If they are also affected, the issue is with the supply rather than the boiler itself.
- Faulty or worn ignition electrode Sometimes
The ignition electrode creates the spark that lights the burner. Over time, the ceramic insulator can crack or the tip can become corroded, producing a weak or absent spark. A failed electrode is a well-documented cause of repeated ignition attempts ending in E01.
- Stuck or faulty gas valve Sometimes
The gas valve controls the flow of gas to the burner. If it has seized due to age and wear, or an internal component has failed, it will not open on demand. The boiler's PCB detects no flame and locks out. This is an internal gas-side component and must only be inspected or replaced by a Gas Safe registered engineer.
- Blocked or restricted flue Rare
Keston boilers use a twin-flue design, and a blockage — debris, bird nesting material, or ice — in either the flue outlet or the air intake can prevent the boiler from establishing a stable flame. The fan may also have failed, preventing adequate air flow for combustion.
- Faulty PCB or flame-sensing circuit Rare
If the PCB is not sending the correct signal to the ignition components, or the flame-detection circuit is failing to register a flame that is actually present, the boiler will lock out after five attempts. PCB faults are relatively uncommon but are more likely on older or poorly serviced appliances.
How to fix it
- Check other gas appliances in the property DIY safe
Turn on a gas hob ring or other gas appliance. If nothing else is working, you may have a supply issue — contact your gas supplier or National Gas (0800 111 999 if you smell gas). If other appliances work, the gas supply is likely fine and the fault is with the boiler.
- Check for a frozen condensate pipe DIY safe
In cold weather, locate the white plastic condensate pipe that exits the boiler — usually through an outside wall — and runs to a drain. If it feels solid or has visible ice on it, this is almost certainly the cause. Pour warm (not boiling) water from a kettle along the frozen section, working from the boiler end outward, until you can hear water flowing freely again. Do not use a naked flame or boiling water.
- Reset the boiler DIY safe
Press and hold the reset button on the Keston C36 control panel as directed in your user guide. The boiler should attempt to relight. If it fires up and runs normally, monitor it over the next hour to confirm it stays on. Avoid resetting more than two or three times in quick succession — repeated resets without resolving the underlying fault will not help and may mask a more serious problem.
- Top up system pressure if low DIY safe
While not a direct cause of E01, low system pressure can contribute to boiler instability. Check the pressure gauge on the boiler — it should read between 1.0 and 1.5 bar when cold. If it is below 1 bar, use the filling loop to top it up carefully, then re-attempt a reset. If you are unsure how to do this, refer to your boiler's user guide or watch a manufacturer-approved video first.
- Call a Gas Safe registered engineer with Keston experience Gas Safe engineer
If the boiler continues to lock out after your checks, the fault requires professional diagnosis. An engineer will test gas pressure at the valve, inspect the ignition electrode and lead, check the fan's speed and output, and assess the PCB. Because Keston boilers have a twin-flue arrangement and a modulating pump, not every engineer is familiar with them — it is worth asking when you book whether they have experience with Keston appliances. Always verify your engineer's Gas Safe registration at gassaferegister.co.uk before work begins.
Parts you may need
- Ignition electrode · from £35
- Ignition lead · from £25
- Gas valve · from £180
- Fan / air pressure switch assembly · from £120
- PCB (printed circuit board) · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£380, depending on the underlying cause.
Frequently asked questions
Why does my Keston C36 keep showing E01 in cold weather?
The most likely culprit in cold weather is a frozen condensate pipe. The external section of this pipe can ice up overnight, stopping the boiler from expelling waste water and causing it to lock out. Thawing the pipe with warm water and resetting the boiler usually resolves it within minutes. If it keeps freezing regularly, ask an engineer about lagging the pipe or rerouting the external run to reduce exposure.
Can I reset the E01 fault myself, and how many times?
Yes — pressing the reset button on the control panel is a homeowner-safe action. However, limit yourself to two or three attempts. If the boiler relights but locks out again shortly afterwards, there is an underlying fault that resetting will not cure. Continuing to reset can mask the problem and, in rare cases, allow unburnt gas to accumulate.
How much does it typically cost to fix a Keston E01 fault?
For the most common repairs — a service visit to clean or replace the ignition electrode, clear a flue obstruction, or resolve a gas pressure issue — most homeowners pay between £120 and £380 including parts and labour. A gas valve replacement sits toward the higher end of that range. If the PCB has failed, costs can rise to £500 or more, and on an older boiler it may be worth getting a replacement quote alongside any repair estimate.
Do I need a specialist engineer for a Keston boiler, or will any Gas Safe engineer do?
Any Gas Safe registered engineer is legally qualified to work on a Keston boiler. However, Keston appliances use a twin-flue design and a modulating pump that differ from more common boiler types, and some engineers are less familiar with their fault-finding procedures. It is worth asking when you book whether the engineer has worked on Keston boilers before — one who has will typically diagnose the fault more quickly and confidently.